With the increase in the size of a liquid crystal display cell in recent years, so-called liquid crystal dropping method has been proposed as a process for producing a liquid crystal display cell higher in mass productivity (see Japanese Patent Application Laid-open Nos. 63-179323 and 10-239694). Specifically, it is a process for producing a liquid crystal display cell comprising dropping liquid crystals inside a sealant for liquid crystals formed on a substrate and then attaching another substrate thereto thereby sealing the liquid crystals.
However, the liquid crystal dropping method has a problem that a sealant for liquid crystals in an uncured state contacts the liquid crystal at first and at that time the ingredients of the sealant for liquid crystals dissolve in the liquid crystals, which causes defect of reducing the specific resistance of the liquid crystal and therefore the method has not been fully spread as a mass-production method of liquid crystal display cell.
There have been contemplated three methods in the liquid crystal dropping method, i.e., a heat-curing method, a photo-curing method and a photo-and-heat-curing method as a method for curing a sealant for liquid crystals after attaching the substrates together. The heat-curing method has problems that the liquid crystals expanded by heating leak out of the sealant for liquid crystals which is lowered in viscosity in the middle of curing and that ingredients of the sealant for liquid crystals which is lowered in viscosity dissolve in the liquid crystals, and these problems are difficult to solve. Therefore, the heat-curing method is not yet put in practical use.
On the other hand, the sealant for liquid crystals used for the photo-curing method includes two types, cationic polymerization type and radical polymerization type depending on the type of photopolymerization initiator. As for the cationic polymerization type sealant for liquid crystals, there is a problem that ions are generated at the time of photo-curing and when it is used in the liquid crystal dropping method, the ionic ingredients elute in the liquid crystals in contact therewith and decreases the specific resistance of the liquid crystals. In addition, since the curing contraction of the radical polymerization type sealant for liquid crystals at the time of photo-curing is large, there is a problem that adhesion strength is not sufficient. Furthermore, as a problem common in the photo-curing methods of both the cationic polymerization type and the radical polymerization type, there is a problem that shaded parts which are not irradiated with light are resulted in the sealant for liquid crystals due to metal wiring of the array substrate of liquid crystal display cell or black matrix of color filter substrate and such shaded parts remain uncured.
In this way, various problems are involved in the heat-curing method and the photo-curing method, and actually photo-and-heat-curing method is considered to be the most practical method. The photo-and-heat-curing method is characterized in that after the sealant for liquid crystals placed between the substrates is irradiated with light to perform primary curing, it is heated to perform secondary curing. As properties required of the sealant for liquid crystals used for the photo-and-heat-curing method, it is important that the sealant for liquid crystals does not contaminate the liquid crystals in each step before and after the light irradiation and before and after the heat curing. Particularly needed are measures to deal with the problem by shaded parts described above, i.e., measures to deal with the elution of the ingredients of the sealant into the liquid crystals at the time of heat-curing from the parts which are not photo-cured. As a solution therefor, there may be considered (i) an approach in which rapid curing is performed at a low temperature before the ingredients of the sealant elute, or (ii) an approach to constitute the sealant with ingredients which hardly elute into the liquid crystal composition, and so on. Naturally, however, rapid curing at a low temperature concurrently means that the pot life at the time of use is deteriorated, and poses a practically large problem. Therefore, in order to attain a sealant for liquid crystals having a long pot life and achieving low contamination of liquid crystal, it is necessary to constitute the sealant with ingredients which hardly elute into the liquid crystal composition. However, since epoxy resins generally known well, for example, bisphenol A epoxy resin and bisphenol F epoxy resin have good compatibility with liquid crystal, it is hard to say that they are suitable as ingredients of a sealant composition from a viewpoint of contaminating properties.
Japanese Patent Application Laid-open No.2001-133794 proposes to use a partially (meta)acrylated bisphenol A type epoxy resin described in Japanese Patent Application Laid-open No.5-295087 as the main resin ingredient for a sealant for liquid crystals for dropping method (see Japanese Patent Application Laid-open Nos. 2001-133794 and No. 5-295087). However, the solubility to the liquid crystal is reduced but unsatisfactorily by (meta)acrylation, and it is also difficult to solve the problem that the unreacted and remained epoxy resin material contaminates the liquid crystal.
As explained above, photo-and-heat-curing type sealant for liquid crystals for a liquid crystal dropping method conventionally proposed are not satisfactory in all of the properties such as liquid crystal contaminating properties, adhesion strength, usable life at room temperature, and low-temperature curing properties.